The invention relates to a double pusher centrifuge including a screen drum rotatable about an axis of rotation for separating a mixture into a solid material cake and a liquid phase, a mixture distributor arranged in the screen drum and having a pusher base device, which is arranged to be movable along the axis of rotation, so that the solid material cake is alternately displaceable with an outer ring zone, and with an infeed device with which the mixture can be introduced via the mixture distributor into an empty space, which forms adjacent to the outer ring zone on displacement of the solid material cake by the pusher base device.
Centrifuges in the most diverse designs for drying damp substances or damp mixtures of substances are very widely known and are employed in the most diverse fields. Thus for example discontinuously operating centrifuges, such as peeling centrifuges, are preferably used for drying high purity pharmaceutical products, whereas continuously operating pusher centrifuges are advantageously used in particular when large amounts of a congealed liquid mixture are to be separated continuously. In this context one-stage or multi-stage pusher centrifuges and also so-called double pusher centrifuges are used depending on the requirements.
In the different types of the last-named class of pusher centrifuges, a solid/liquid mixture, for example a suspension or a moist salt or salt mixture, is fed through an infeed tube via a mixture distributor of a rapidly rotating drum, which is designed as a filter screen, so that the liquid phase is separated through the filter screen due to the centrifugal forces that are operating while a solid material cake is deposited in the inside on the drum wall. In this arrangement a substantially disc-shaped synchronously co-rotating pusher base, which oscillates in the axial direction with a certain amplitude, is arranged in the rotating drum, so that one part of the dried solid material cake is pushed out at one end of the drum. In the opposite movement of the pusher base, an adjacent region of the drum is exposed, which can then be fed with a new mixture through the infeed tube and via the mixture distributor again. In this arrangement, depending on the type used, throughput amounts of the order of magnitude of 100 tons per hour can be achieved with modern high-performance pusher centrifuges, with drum diameters of up to 1000 mm and more being common. Typical rotational frequencies of the drum of up to 2,000 rotations per minute and more can be reached, depending on the drum diameter. As a rule a larger drum diameter implies a smaller maximum rotational frequency of the drum, due to the strong centrifugal force which occurs. The operating parameters, such as for example the rotation frequency of the drum, the amount of mixture added per unit of time or also the drum diameter, or the type of pusher centrifuge used, can also depend on the material to be dried, on the content of liquid, etc.
In the known double pusher centrifuges the mixture usually reaches the middle of the centrifugal drum via a stationary infeed tube and a mixture distributor, wherein the mixture distributor rotates in synchronism with the centrifuge drum. The mixture can be fed, in cooperation with the mixture distributor, alternately to the front or back drum half, by a pusher base arranged in the middle of the centrifuge drum, which oscillates along the longitudinal axis of the centrifuge drum and which can be operationally connected to the mixture distributor. As a result of this, two inflow zones are present, so that correspondingly large amounts of mixture can be processed per unit of time. In this arrangement, the solid material cake is transported by the pusher base to the respective end of the drum and carried out via a collecting channel.
A known double pusher centrifuge, which operates in accordance with the principle described above, is described in detail in EP 0 635 309 B1. The advantages over conventional one-stage or multi-stage pusher centrifuges are obvious. Among other things the double inflow zone is to be named, through which a clearly increased liquid swallowing capacity is achieved; i.e. it is possible to work with a higher liquid content. At the same time higher total feed amounts of mixture can be processed. Furthermore, the double solid material feed capacity is achieved for the same stroke number and thus a specifically lower transport power. In this arrangement the space requirements correspond to those of normal pusher centrifuges of the same constructional size.
Typical fields of operation for double pusher centrifuges are, among others, products which can be readily dewatered, such as sea salt for example, where in particular the double exploitation of the pusher movement comes into full effect. A further typical field of use is products which can be filtered poorly or mixtures with low inflow concentrations (i.e. with a high liquid content). Here the swallowing capacity which is high in comparison with usual pusher centrifuges has a particularly positive effect. Smaller inflow concentrations or higher suspension amounts can be processed without resulting in flooding.
However, known pusher centrifuges also display various grave disadvantages. Even if lower infeed concentrations can be processed with the known double pusher centrifuges than with usual one-stage or multi-stage pusher centrifuges, the infeed concentration of the mixture which is to be processed cannot be indefinitely small. I.e. if the proportion of liquid in the mixture is too high, for example amounting to 50% or 70% or 80% or even more than 90% liquid phase, the mixture has to be pre-thickened using more or less complicated processes. If the liquid content is too high a regular distribution of the mixture to be dried over the extent of the screen drum is increasingly difficult. On the one hand this can lead to extremely damaging vibrations of the screen drum and thus to premature wear and tear on the bearings and the drive; in the worst case it can even lead to a safety problem in operation. On the other hand, an irregular solid material cake distributed irregularly over the circumference of the screen drum can lead to problems during washing. For this reason, static thickeners, curved screens or the well-known hydrocyclones are available. It is obvious that the use of preliminary drying systems is very complicated to operate, both from the point of view of the technical process and the apparatus required, and is thus expensive.
A further grave disadvantage in the processing of mixtures of small infeed concentration is that practically the whole amount of liquid which is fed in with the mixture has to be accelerated to the full peripheral speed before it is separated out through the filter screen of the screen drum. The same applies to the smallest particles in the mixture which are likewise to be separated through the screen from the solid material cake. This is extremely undesirable from an energy point of view and has a clearly negative influence on the operating behavior of the centrifuge.
Even in the processing of mixtures with considerably higher solid material concentration some of the centrifuges known from the prior art show massive disadvantages. Thus the mixture introduced into the mixture distributor through the infeed tube is accelerated to the full peripheral speed of the drum in a very short time after reaching the screen drum. In particular, in the case of sensitive substances this can lead to granule breaking. This means, for example, that solid material granules which are distributed in a suspension which has been introduced to the centrifuge burst apart into smaller pieces in an uncontrolled manner during the abrupt acceleration process, which can have a negative influence on the quality of the solid material cake which has been produced, for example, if the particle size of the granules in the end product is important.